Thermally sprayed, flexible magnet with an induced anisotropy

ABSTRACT

Disclosed is a process for making a flexible magnet with an induced anisotropy, and in particular to a process for making a flexible anisotropic magnet by thermal spraying in the presence of an applied magnetic field. The method may be used to fabricate a substrate having a flexible anisotropic magnetic coating or a free standing anisotropic flexible magnet.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention generally relates to flexible magnets withan induced ANISOTROPY, and in particular to flexible anisotropic magnetsmade by thermal spraying.

BACKGROUND OF THE INVENTION

[0002] Flexible magnets are used widely in electromechanical devices,e.g., generators, relays, motors, and magnetos; electronic applications,e.g., loudspeakers, travel-wave tubes, and telephone ringers andreceivers; antitheft tags; holding devices, such as door closers, seals,and latches; and magnetic recording devices. Flexible magnets have beenwidely used in many applications because of desirable properties, suchas good plasticity or resiliency and superior workability. Thesedesirable properties are not found in hard magnets, such as sinteredferrite magnets or alloy magnets. However, the magnetic properties ofsuch magnets have not been satisfactory because they are generallyproduced by blending a pulverized magnetic material with a rubber orplastic matrix. For example, prior art flexible magnets generally do nothave a high enough energy product, i.e., the product of the coercivityand the remnant magnetization, which necessitates the use of largermagnets than that of the conventional sintered magnet for the sameapplication. Accordingly, applications for flexible magnets have beenrestricted.

[0003] Furthermore, prior art flexible magnets are typically made bymixing substantially domain-size particles of a hexaferrite with aflexible binder and then shaping the mixture, typically by extrusion.The resulting free standing flexible magnets are limited in shape orform to long strips that must be cut down to size for practical use. Inaddition, flexible magnets produced from such processes can only beattached to a surface/substrate by undergoing another production step ofusing an additional fixing agent, such as an adhesive. Lastly, prior artflexible magnets are produced by using volatile organic compounds(VOC's) as the solvent. The use of such VOC's are environmentallyhazardous, and the presence of VOC's is not desirable during theproduction process or in the final product.

[0004] The critical factors for improving magnetic properties offlexible magnets are as follows: (1) maximizing the magnetic particulatecontent in the matrix material; (2) maximizing the orientation of themagnetic particles in the matrix material in a desired direction; and(3) maximizing the energy product, i.e., the product of the coercivityand the remnant magnitization.

[0005] Accordingly, there exists a need in the art for a cost-effectivemethod for efficiently making a flexible magnet having (i) an inducedmagnetocrystalline anisotropy, and (ii) complex geometric shapes whichcannot be achieved by an extrusion process. There also exists a need foran efficient method to provide a substrate with a flexible anistropicmagnetic coating without the need for adhesives. Finally, there alsoexists a need for a substantially VOC free process for making flexiblemagnets.

SUMMARY OF THE INVENTION

[0006] The present invention encompasses a method for producing aflexible anisotropic magnetic coating onto a substrate. The methodincludes the step of thermal spraying a first spray stream of compositeparticles, which include magnetic particles incorporated into or onto amatrix material. The thermal spraying step is conducted at a temperaturethat is above the glass transition or melting point temperature of thematrix material, and a magnetic field is applied across the substrate.In one embodiment, the method further includes at least one additionalspray stream of a magneto-fluid mixture. The at least one additionalspray stream is combined with the first spray stream to coat thesubstrate. These novel methods provide magnetically coated substrateswhich exhibit magnetocrystalline anisotropy.

[0007] In another embodiment, a flexible, free standing, complexthree-dimensional anisotropic magnet is provided by substituting thesubstrate with a removable mold in the above-described method. Theseflexible anisotropic magnets have magnetic particles dispersed within amatrix material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Further objects and advantages of the present invention will bemore fully appreciated from a reading of the detailed description whenconsidered with the accompanying drawings wherein:

[0009]FIG. 1 illustrates a thermal spray arrangement according to thepresent invention;

[0010]FIG. 2 illustrates an alternative thermal spraying arrangementaccording to the present invention;

[0011]FIG. 3 illustrates a typical mechanofusion apparatus;

[0012]FIG. 4 shows SEM micrographs composite particles obtained from amechanofusion milling process according to the present invention;

[0013]FIG. 5 illustrates a thermal spray arrangement according to thepresent invention which also includes a Suspension Atomizing System;

[0014]FIG. 6 illustrates a Suspension Atomizing System;

[0015]FIG. 7 illustrates a hysteresis loop for a flexible magnetaccording to the present invention having 12% by volume of strontiumferrite in a polyethylene-methacrylic acid copolymer matrix according tothe present invention;

[0016]FIG. 8 illustrates a hysteresis loop for a flexible magnetaccording to the present invention having 20% by volume of strontiumferrite in a polyethylene-methacrylic acid copolymer matrix;

[0017]FIG. 9 illustrates a comparison between the hysteresis loops for aflexible magnet formed in a parallel applied field and in aperpendicular applied field, respectively, according to the presentinvention, the flexible magnet having 8% by volume of strontium ferritein a polyethylene-methacrylic acid co-polymer matrix;

[0018]FIG. 10 shows the X-ray diffraction pattern for Part a of FIG. 1for a flexible magnet according to the present invention having 8% byvolume of strontium ferrite in a polyethylene-methacrylic acidco-polymer matrix;

[0019]FIG. 11 shows the X-ray diffraction pattern for Part b of FIG. 1for a flexible magnet according to the present invention having 8% byvolume of strontium ferrite in a polyethylene-methacrylic acidco-polymer matrix; and

[0020]FIG. 12 illustrates hysteresis loops for a flexible magnet formedin a perpendicular magnetic field and in a parallel magnetic field,respectively, according to the present invention, the flexible magnethaving 38% by volume of strontium ferrite in a polyethylene-methacrylicacid co-polymer matrix.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention provides a method for producing a flexiblemagnet having induced magnetocrystalline anisotropy energy. The flexiblemagnets according to the present invention have high energy productsbecause (i) a high concentration of magnetic particles is incorporatedinto the matrix material and (ii) a high degree of orientation of themagnetic particles in the matrix material. High energy product, as usedherein, means that the product of the coercivity and the remnantmagnitization is greater than about 1.2 MG Oe, preferably greater thanabout 2.9 MG Oe, and most preferably greater than about 3.5 MG Oe. Theterm “about,” as used herein means ±10% of the stated value.

[0022] The method of producing a flexible anisotropic magnet accordingto the present invention includes the step of thermal spraying acomposite mixture, which includes composite particles of a matrixmaterial and magnetic particles, onto a substrate at a temperature thatis above the glass transition/melting point temperature of the matrixmaterial but below the Curie temperature of the magnetic particles. Thethermal spraying step is conducted while applying a magnetic fieldacross the substrate. The present invention also provides an article ofmanufacture having magnetocrystalline anisotropic energy, which includes(i) a substrate, and (ii) a flexible anisotropic magnetic coatingfixedly attached to the substrate. Since the present method does notrequire solvents, the flexible anisotropic magnets of the presentinvention are substantially free of volatile organic compounds (VOC's).Substantially free, as used herein, means that the flexible anisotropicmagnets of the present invention have less than about 10%, preferablyless than 5%, and most preferably less than 1% by weight of thereferenced component. As a result, the flexible anisotropic magnets ofthe present invention and the process for making these magnets areenvironmentally friendly.

[0023] The present invention also provides a flexible anisotropic magnethaving complex three-dimensional spray mold forms. In this embodiment,the substrate becomes a mold or work piece. Such substrate molds have anon-stick surface, typically including a fluoronated surface of apolymer, such as TEFLON. Complex three-dimensional spray mold forms, asused herein, means three-dimensional shapes which are created by using amold, such as those used in manufacturing injection molded plasticproducts, to obtain any desired three-dimensional shape. Thesethree-dimensional shapes cannot be obtained by using only an extrusionprocess, i.e., another process step would be required to obtain thedesired shape.

[0024] Surprisingly, it has been found that the flexible magnets andflexible magnetic coatings produced according to the present inventionhave induced magnetocrystalline anisotropy, i.e., the ability to orientnearly all their important magnetic properties, such as remanence—B_(r),coercivity—H_(c), and maximum energy product—(BH)_(max), in a particulardirection. Furthermore, the method of producing flexible anisotropicmagnets in accordance with this invention is (1) efficient because thereis little or no loss of the magnetic particles and the matrix material,(2) cost effective because the steps are low in cost, and (3)environmentally friendly since no volatile organic compounds are needed.

[0025] The novel method of producing a flexible anisotropic magneticcoating according to the present invention can be practiced using theThermal Spraying Apparatus illustrated in FIG. 1. A composite mixture isthermally sprayed by a commercially available thermal spray gun 10 ontoa substrate 20 to form a flexible anisotropic magnetic coating 30 on topof the substrate while a permanent magnet 40 produces a magnetic fieldacross the substrate. The thermal spray gun 10 can produce a minimumfilm thickness of about 100 microns. Commercially available thermalspray guns include model PFS400 available from Plastic FlamecoatSystems, located in Big Spring, Tex. The duration of the spray onto aparticular region of the substrate can be increased to produce a desiredthickness of the flexible anisotropic magnetic coating, i.e., athickness to about 2 cm. Alternatively, a desired thickness can beobtained by repeating the thermal spraying step over a particularregion.

[0026] During the spraying step, a permanent magnet 40, which is placedbehind the substrate, produces a magnetic field across the substrate.Although in one embodiment the pole pieces are in contact with thesubstrate, it would be clear to one skilled in the art that the distanceof the pole pieces to the substrate can be appropriately adjusted. Inother words, the permanent magnet is positioned at an appropriatedistance from the substrate so as to induce a magnetic field ofsufficient strength to obtain the desired orientation of the magneticparticles during the thermal spray step. The substrate can be virtuallyany surface, e.g., solid, semi-solid, porous, or non-porous. Themagnetic field is set to be strong enough to orient the magneticparticles in the heated matrix material layer before the matrix materialsolidifies on top of the substrate. The magnetic field of the permanentmagnet is preferably in the range of about 9000 Oe to about 11,000 Oe atthe pole piece. The magnetic field lines have a perpendicular or normalcomponent to the coating surface at region a and a parallel component tothe coating surface at region b. One of ordinary skill in the art wouldappreciate that more than one permanent magnet can be placed behind thesubstrate in various configurations to obtain numerous regions ofparallel and normal magnetic field lines. It is believed that theindividual magnetic particles that are thermally sprayed alignthemselves according to the magnetic field of the permanent magnet 40placed behind the substrate 20 while the heated matrix material is in afluid or semi-fluid state. This novel method produces a flexibleanisotropic magnetic coating which becomes fixedly attached onto thesubstrate without the need for any additional adhesives.

[0027] In another embodiment, the substrate can be a removable mold orwork piece on which or in which a free standing flexible magnet can beformed. In other words, the flexible magnet conforms to the shape of themold. Such substrate molds have a non-stick surface, typicallycomprising a fluoronated surface of a polymer, such as TEFLON, and arewell known to one skilled in the art of manufacturing molded plasticproducts. These substrate molds allow the flexible anistropic magnets ofthe present invention to be formed into any desired complexthree-dimensional shape, such as solenoids, spheres, large cylinders,ellipses, or any other desired shape, without additional costlyproduction steps, such as extrusion and cutting. The term “complexthree-dimensional shape,” as used herein means any three-dimensionalform or shape which cannot be obtained using only an extrusion process.

[0028] The thermal spray gun 10 includes a heating means and a sprayingmeans, which are not illustrated in FIG. 1. The heating means istypically a temperature controlled flame torch, which is positionedadjacent to the outlet of the spraying means. Alternatively, the heatingmeans can heat a carrier gas which is used to heat the composite mixturecarried by the gas. The spraying means typically includes a nozzlethrough which the composite mixture is pumped using a carrier gas,typically ambient air applied at positive pressure. When metallicmagnetic particles are used, an inert carrier gas, such as nitrogen, ispreferred. Typically, the heating means is set at a temperature that ishigher than the melting point or glass transition temperature of thematrix material but lower than the Curie temperature of the magneticparticles. The inlet to the spraying means is fluidly connected to apumping means (not illustrated) which combines a first feed line for acarrier gas, typically ambient air or an inert gas, with a second feedline which is connected to a reservoir (not illustrated) containing thecomposite mixture. Such a thermal spray gun apparatus has been used forthermal spraying of various polymers, as described in J. A. Brogan, “Thecoalescence of combustion-sprayed ethylene-methacrylic acid copolymer,”Journal of Materials Science, Vol. 32(8) pp. 2099-2106 (1997), which isincorporated herein by reference.

[0029] Another embodiment of the thermal spray apparatus is illustratedin FIG. 2. In this embodiment, a permanent magnet or an electromagnet 40is mounted with its pole faces positioned on opposing sides of theoutlet of the thermal spray gun 10 above a horizontally moving substrate20. The pole pieces of the magnet are positioned to produce a magneticfield having a parallel component to the substrate 20 and thereby orientthe magnetic particles in the fluid/semi-fluid matrix material. Theresult is an anisotropic magnetic coating 30 that is fixedly attached tothe substrate 20. Such a thermal spraying apparatus is useful when amagnet cannot be placed behind the substrate, e.g., when providing ananisotropic magnetic coating onto a road.

[0030] The reservoir (not shown) contains a composite mixture whichincludes composite particles of a matrix material and magnetic particlesbound to the matrix material. Preferably, the composite mixture furtherincludes matrix material particles which do not have magnetic particlesbound thereto and/or therein. The composite particles of the magneticparticles and the matrix material have an average particle size fromabout 20 microns to about 200 microns. These composite particles areobtained by introducing the magnetic particles into the host matrixmaterial particle by any method well known to one skilled in the art.Preferably, the magnetic particles are introduced into or onto the hostmatrix material particle using a process known in the art asmechanofusion, as described in Tohei Yokoyama, “The AngmillMechanofusion System and Its Applications,” KONA, No. 5 pp. 59-68(1987), which is incorporated herein by reference. FIGS. 3(a) and 3(b)provide an illustration of a typical mechanofusion system. FIG. 3(a)provides a top view, and FIG. 3(b) provides a crosssectional side view.The magnetic particles and the matrix material are added into thechamber 60 of the mechanofusion apparatus 50. A scrapper 80 and an innerpiece 85 are separately attached to a rotating shaft 70 via arms 75. Asthe scrapper 80 and inner piece 85 are rotated against the chamber wall61 within the chamber 60, the inner piece 85 subjects mechanical forceto the mixture of magnetic particles and the matrix material to form alayer of composite particles. The layer of composite particles are thanscrapped off of the chamber wall 61 by the scrapper 80. An example of acommercially available mechanofusion apparatus is model AF-15, availablefrom Hosokawa Company, located in Summit, N.J.

[0031] It is believed that the mechanofusion process helps to (i)disperse the magnetic particles by incorporation into or onto the matrixmaterial particles so that the magnetic particles behave as singledomains (i.e., agglomeration of the magnetic particles is prevented byminimizing exchange-coupling between individual magnetic particles), and(ii) produce a more spherical composite particle to enhance flow throughthe thermal spray gun assembly. FIGS. 4(a) and 4(b) provide scanningelectron micrographs (SEM) of composite particles obtained from themechanofusion process at 50× and 200× magnification, respectively. Themicrographs show small magnetic particles (light areas) bound to thelarge matrix material particles (dark areas).

[0032] Magnetic particles that are useful according to the presentinvention are hard magnetic materials that are generally characterizedby high coercivity (H_(c)), high remanent induction (B_(r)) and highmaximum energy product ((BH)_(max)), as described in Kirk-OthmerEncyclopedia of Chemical Technology, 4^(th) Ed., Vol. 15, pp. 723-788(John Wiley & Sons, 1995), which is incorporated herein by reference.These magnetic particles have an H_(c) greater than about 150 Oe,preferably greater than about 2,000 Oe, and are preferably in a singledomain state. The magnetic particles have an average particle size fromabout 1 micron to about 10 microns, preferably from about 2 microns toabout 5 microns. When the magnetic particles are metallic materials, itis preferable to use oxygen-free spraying conditions. For example,nitrogen gas may be used as the carrier gas for the thermal sprayassembly.

[0033] Suitable magnetic particles that are useful according to thepresent invention include, but are not limited to, hard ferrites;rare-earth R—Co alloys; isotropic or anisotropic, high H_(c), andcolumnar Alnicos; ternary R-based magnetic materials;chromium-cobalt-iron alloys; copper-nickel-iron and copper-nickel-cobaltalloys; platinum-cobalt alloys; manganes-ealuminum-carbon alloys; andmixtures thereof.

[0034] Hard ferrites that are useful according to the present inventionare typically characterized by the general formula MO.6Fe₂O₃ where M isBa or Sr. Examples of hard ferrites include, but are not limited to,SrFe₁₂O₁₉ (T_(c)=450° C.) and BaFe₁₂O₁₉ (T_(c)=450° C.).

[0035] Rare-earth R—Co alloys that are useful according to the presentinvention are typically characterized by the general formula RCo₅ whereR is a rare-earth transition metal, preferably selected from the groupconsisting of Ce, Pr, Nd, and Sm. Examples of rare-earth R—Co alloysinclude, but are not limited to, SmCo₅ (T_(c)=730° C.), CeCo₅(T_(c)=374° C.), PrCo₅ (T_(c)=612° C.), NdCo₅ (T_(c)=630° C.),Sm(Co_(0.68)Cu_(0.10)Fe_(0.21)Zr_(0.01))_(7.4) (T_(c)=800° C.), andSm₂Co₁₇ (T_(c)=920° C.).

[0036] Ternary R-based magnetic materials that are useful according tothe present invention include, but are not limited to, Nd₂Fe₁₄B(T_(c)=312° C.), Nd₂Fe₁₄C (T_(c)=262° C.), Nd₂Fe₁₄N (T_(c)=312° C.),Fe₃B:Nd (T_(c)=512° C.), SmFe₁₁Ti (T_(c)=312° C.), SmFe₁₀V₂ (T_(c)=337°C.), SmFe₁₀Mo₂ (T_(c)=187° C.), Sm₂Fe₁₇C (T_(c)=267° C.), Sm₂Fe₁₇N_(2.7)(T_(c)=477° C.), Sm(Co,Fe,Cu)₇ (T_(c)=827° C.), and Nd₂Co₁₄B (T_(c)=722°C.).

[0037] Cobalt ferrites are also useful as magnetic particles accordingto the present invention. Preferred are cobalt ferrites in which some ofthe cobalt and some of the iron is substituted by other transition metalions, provided that at least 50% of the divalent metal is cobalt. Anexample of useful cobalt ferrites include, but is not limited to,CoFe₂O₄ (T_(c)=520° C.).

[0038] Matrix materials that are useful according to the presentinvention are amorphous or crystalline polymers which have a sharpchange in viscosity at its glass transition temperature or meltingpoint, respectively, so that the matrix material can be converted to afluid/semi-fluid state in a relatively short period of time, i.e.,become fluid/semi-fluid during the time it is heated in the thermalspray assembly. In addition, the glass transition temperature/meltingpoint must be lower than the Curie temperature of the magneticparticles. Those skilled in the art would appreciate that a conventionalpre-heating processes can be used to enhance phase transitions over ashort period of time. This phase transition from a solid to afluid/semi-fluid is essential to allow the individual magnetic particlesto be oriented according to the applied magnetic field while the matrixmaterial is in the fluid/semi-fluid state, i.e., before the matrixmaterial cools and solidifies. The matrix material, which is provided inparticulate form, may have an average particle size from about 30microns to about 250 microns, preferably from about 40 microns to about180 microns. Typically, the matrix material particles are about 20-60times larger on average than the magnetic particles.

[0039] Matrix materials useful according to the present inventioninclude, but are not limited to, polyethylene; polyethylene-methacrylicacid copolymer (EMAA); polypropylene; polyvinylchloride;polyvinylacetate; nylon; ABS; polycarbonate; polystyrene; methacrylresin; polyacetal; polyamide resin; thermoplastic polyurethane; EVAresin; polysulfone (commercially available from Amoco and ICI);polyether sulfone (commercially available from Amoco and ICI);polyarylsulfone (commercially available from Amoco and ICI);polyetherimide; imide-based polymers (commercially available fromGeneral Electric and Hoechst-Celanese); polyphenylene oxide;fluoroplastics; acrylonitrile-styrene resin; ionomer resin;vinylchloride-vinylacetate copolymer; chlorosulfonated polyethylene(commercially available as HYPALON 450); polyisobutylene (commerciallyavailable as VISTANEX L-140); ketone-based polymers such as polyketone(commercially available as Kadel from Amoco), poly(etherketone)(commercially available as Hostatec from Hoechst, UltraPek from BASF,and VictrexPek from ICI); poly(etheretherketone) (commercially availableas Victrex from ICI); poly(etherketoneketone) (commercially available asPEKK from Du Pont); poly(phenylene sulfide) (commercially available asRyton from Phillips; Tedur from Bayer; Supec from General Electric; andFortron from Hoechst); and mixtures thereof.

[0040] Matrix materials most useful according to the present inventioninclude, but are not limited to, the low temperature plastics (LTP's)that exhibit low melt viscosities. LTP's include polyethylene (e.g.,Alkathene™, commercially available from ICI, located in New York, N.Y.),polypropylene (e.g., Novolen™, commercially available from BASF, locatedin Clemson, S.C.), polyester elastomers (e.g., Hytrel™, commerciallyavailable from Dupont Company, located in Wilmington, Del.) andpolyethylene copolymers and ionomers. Polyethylene copolymers such asethylene methacrylic acid copolymer (EMAA) is commercially availablefrom Dupont Company as Nucrel™ and the ionomer based upon EMAA is alsocommercially available from Dupont Company as Surlyn™. Otherpolyethylene copolymers of interest include ethylene acrylic acidcopolymer (EAA) and ethylene vinyl alcohol copolymer (EVA). Polymerresins that have melt-flow indices in the range of 7 to 700 provide aneffective matrix for the magnetic 2 ^(nd) phase. A higher melt flowindex corresponds to a lower molecular weight and melt viscosity, whichwill allow greater magnetic orientation during deposition.

[0041] In one embodiment of the present invention, additional matrixmaterial particles, which are free of magnetic particles, can be addedto the composite particles obtained from the mechanofusion process. Thisis typically done when the composite particles fail to act as a flowablepowder mixture as a result of magnetic agglomeration. Typically, thecomposite particles are mixed with additional matrix material particlesat a ratio of 1:4 to 10:1, preferably 2:3 to 9:1, respectively byvolume. The additional matrix material is the same as that describedabove for the matrix material.

[0042] The total volume percentages of the magnetic particles and thematrix material in the composite mixture, e.g., the composite particlesand the matrix material particles, are chosen so that the compositemixture is a flowable powder mixture that can be pumped continuouslythrough the feeder and the spray nozzle. As a result, the practicalupper limit for magnetic particulate percentage by volume is the volumepercentage at which the composite particles magnetically agglomerate,thereby preventing continuous flow through the feeder and the spraynozzle. Although the practical limit for magnetic particulate volumepercentage differs for specific magnetic particles and specific matrixmaterials, the upper limit for a composite particle comprising strontiumferrite (SrFe₁₂O₁₉) and polyethylene methacrylic acid copolymer has beenfound to be about 20% by volume of strontium ferrite.

[0043] In another embodiment of the present invention, the thermal spraystep includes a second spray stream 150 which is introduced by aSuspension Atomizing System (SAS), as described in FIGS. 5 and 6. Thissecond spray stream 150 provides a method of increasing the volumepercentage of the magnetic particles without contributing to theagglomeration problem associated with the first spray stream from thethermal spray gun. The SAS system includes a stirred reservoir 110containing a magneto-fluid mixture. The reservoir is fluidly connectedto a controllable pump 120, which is typically a peristaltic pump. Theoutlet of the pump is fluidly connected to an atomizing probe 140, whichcombines the pumped fluid with an atomizing gas 130. The outlet of theatomizing probe is situated near the outlet of the thermal spray gun.Preferably, the parts of the Suspension Atomizing System that physicallycontact the second fluid stream are made of nonmagnetic materials. Theflow rate of the second spray stream is also preferably chosen so that(i) the direction of the spray stream from the thermal spray gun isnegligibly affected (i.e., the direction of the combined spray streamdoes not deviate more than 20 degrees from the original direction of thefirst spray stream) and (ii) a predominant portion of the magneticparticles, i.e., at least 51%, in the resulting coating or free-standingmagnet is maintained in a single domain state. Typically, the directionof the second spray outlet is set at a 45° angle toward the direction ofthe thermal spray gun outlet.

[0044] The stirred reservoir 110 contains a magneto-fluid mixture of avaporizable fluid, magnetic particles, and a dispersing agent. Suchmagneto-fluid mixtures are well known in the art, since they preventagglomeration of the magnetic particles by effectively dispersing themagnetic particles throughout the vaporizable fluid, as described in G.Schiller et al., “Suspension Plasma Spraying of Cobalt Spinel,”Proceedings of the United Thermal Spray Conference, p. 343 (September1997), which is incorporated herein by reference. The magneto-fluidmixture comprises from about 39.9% to about 60%, preferably from about50% to about 60%, by weight of a vaporizable fluid; from about 39.9% toabout 60%, preferably from about 40% to about 50%, by weight of magneticparticles; and from about 0.1% to about 0.5%, preferably from about 0.2%to about 0.3%, by weight of a dispersing agent. The preferred mixingratio of the magneto-fluid mixture is 1:1:0.4 of vaporizablefluid:magnetic particles:dispersing agent, respectively by weight.

[0045] The magnetic particles are the same as described above. Thevaporizable fluid can be any fluid which (i) provides a uniformdispersion of the magnetic particles, (ii) converts to a gaseous stateat the operating temperatures of the thermal spray gun, and (iii)vaporizes at an operating temperature below the Curie Temperature of themagnetic particles. Typically, the vaporizable fluid is a polar solvent.Vaporizing fluids which can be used according to the present inventioninclude, but are not limited to, water, ethanol, methanol, and mixturesthereof. The dispersing agent typically has surfactant-like propertieswhich help to uniformly disperse the magnetic particles. Dispersingagents which can be used according to the present invention include, butare not limited to, sodium polymethacrylate (30% solution in water),which is sold under the trade name of Darvan No. 7 by R.T. VanderbiltCompany, Inc.

[0046] The methods according to the present invention produce (i)flexible, anisotropic, magnetic coatings on substrates and (ii)flexible, free standing, anisotropic magnets which may have complexthree-dimensional shapes. These magnetic coatings and free standingmagnets have from about 8% to about 38% by volume of magnetic particles.More importantly, the magnetic particles are oriented in a desireddirection so that at least one section of the magnetic coating or freestanding magnet has an easy magnetic axis. Furthermore, these magneticcoatings and free standing magnets have a coercivity that is greaterthan about 2200 Oe.

EXAMPLES

[0047] The following examples further describe illustrative embodimentsof the present invention.

Example 1 Flexible, Anisotropic Magnet Prepared by Thermal Spray

[0048] A flexible, anisotropic magnet having dispersed strontium ferrite(SF) particles (SrFe₁₂O₁₉; Curie temperature=450° C.) in a polyethylenemethacrylic acid copolymer matrix material (EMAA; melting point=95° C.)was made according to the following process. EMAA particles having anaverage particle size of 80 microns was obtained from Plastic FlamecoatSystem located in Big Spring, Tex., and SF particles with an averageparticle size of 2 microns was obtained from Stackpole, Inc., located inKane, Pa. 100 grams of the SF powder was added to 74.8 grams of the EMAAin a mechanofusion milling system (model Angmil AF-15, manufactured byHosakawa located in Summit, N.J.) for 30 minutes at 550 rpm and thenanother 30 minutes at 700 rpm. After each 30 minute interval, themechanofused composite particles were observed under an opticalmicroscope to verify incorporation of the magnetic particles. Assuming adensity of 5.1 gm/cm³ for SF, and 0.93 gm/cm³ for the EMAA, thecorresponding volume percentages for the first sample were approximately20% by volume of SF and 80% by volume of EMAA. The density of the 20% byvolume SF mechanofused composite particles was 1.748 gm/cm³, which wascalculated as follows:

(100 gm SF)/(5.1 gm/cm³)+(74.8 gm EMAA)/(0.93 gm/cm³)=100 cm³ totalvolume,

(100 gm SF+74.8 gm EMAA)/(100 cm³)=1.748 gm/cm³.

[0049] The procedure above was repeated two more times. However, themechanofused composite particles were tumble mixed in a plasticcontainer with additional EMAA particles (free of SF). A 12% by volumeSF sample, the second sample, was made by adding 67 cm³ (62.3 gm) ofEMAA to 100 cm³ (174.8 gm) of the 20% by volume SF mechanofused sample.An 8% by volume SF sample, the third sample, was made by adding 150 cm³(139.5 gm) of EMAA to 100 cm³ (174.8 gm) of the 20% by volume SFmechanofused sample.

[0050] The three samples were individually sprayed onto a Teflon coatedpan using a thermal spray apparatus PFS 200, manufactured by PlasticFlamecoat Systems, located in Big Spring, Tex. The outlet of the spraynozzle was placed 50 inches away from the Teflon coated pan. A permanentmagnet providing a 11000 Oe magnetic field at the pole piece was placedbehind the Teflon coated pan during the thermal spray step. Aftercooling, the magnetic samples were removed from the Teflon coated pan.

[0051] Measurements of the magnetic samples were obtained with aVibrating Sample Magnetometer (VSM), model #1660 manufactured by DigitalMeasurement Systems, Burlington, Mass. The VSM is a highly sensitiveinstrument that is commonly use to accurately measure the magneticproperties of a material. Before measuring the samples, the VSM wascalibrated with a 36.9 mg Ni standard sample having a saturationmagnetization of 2.174 emu. An electromagnet applied a uniform DC(direct current) field of up to 13 kOe to the sample. The resultingmagnetization induced in the sample was then measured by vibrating thesample to produce a voltage in a pair of pickup coils. The coil outputvoltage was combined with the output from the displacement transducer toproduce a magnetization signal. The amplitude and frequency of thevibrations were than canceled out in a signal processor. At a fixedmagnetic field strength applied by the electromagnet, the sample wasmeasured N times (an arbitrary number of data points), and the N valuesof emu were averaged. The process was repeated at a new field strengthuntil a complete hysteresis loop is produced.

[0052] The magnetic sample was positioned in the VSM with the appliedfield aligned in the same direction with respect to the sample as thecomponent of the applied magnetic field parallel to the substrate duringthe thermal spray step. The magnetic samples showed a maximum coercivityof 2275 Oe and saturation magnetization of 9.708 emu/g in thecomposition having 12% by volume of SF, and maximum coercivity of 1965Oe and saturation magnetization of 17.01 emu/g in the composition having20% by volume of SF in the EMAA matrix, as illustrated by FIGS. 8 and 9,respectively.

[0053] The magnetic samples exhibited induced magnetocrystallineanisotropy; that is, the material had a certain easy magnetic axisresulting from the magnetic field applied to the substrate duringthermal spraying. As illustrated in FIG. 9, magnetic data obtained fromVSM measurements for the magnetic sample having 8% by volume SF at part(a) of FIG. 1 (the area of the magnetic sample where the field of theVSM was perpendicular to the substrate surface) and at part (b) of FIG.1 (the area of the magnetic sample where the field of the VSM wasparallel to the substrate surface) showed different magnetic properties.If the magnetic field of the VSM is applied parallel to the direction ofthe applied magnetic field during spraying, the hysteresis loop showsthat the magnetic sample is easier to magnetize, as illustrated byhysteresis curve 9(a). If the field is applied in another direction, thehysteresis loop shows the sample is harder to magnetize, as illustratedby hysteresis curve 9(b). “In another direction” means in a directionother than the direction of parallel and perpendicular components ofmagnetic field applied by the permanent magnet during spraying. Thistest showed that the SF particles are aligned along the applied fielddirection during the spray process. The VSM data was supported by X-RayDiffraction (XRD) data, which showed that the magnetic crystals of SFwere aligned with their easy magnetic axis, the c-axis, along theapplied field direction during thermal spraying, as illustrated by FIG.10 (X-ray diffraction pattern for Part (a) of FIG. 1 for a coatinghaving 8% by volume of SF) and FIG. 11 (X-ray diffraction pattern forPart (b) of FIG. 1 for a coating having 8% by volume of SF). The peaksrepresent atomic planes of hexagonal structure of the strontium ferriteparticles.

Example 2 Flexible Magnets Prepared by Thermal Spray in Combination witha Suspension Atomizing System

[0054] When using the thermal spray system alone, the maximum volumepercentage of SF loaded into the EMAA matrix was 20% due to magneticagglomeration of the feed stock in the feeding mechanism resulting fromthe attractive forces of the magnetic particles in the compositeparticles. The problem of feed stock agglomeration can be solved by (i)using a feeding mechanism the prevents magnetic agglomeration, e.g.,simultaneously providing physical agitation to overcome the attractiveforces, or (ii) reformulating the feed to overcome the attractiveforces, e.g., forming a dispersion having a dispersing aid. Since thepresent thermal spraying system was based on using air as the carrierfluid, a third alternative was used to increase the SF volume percentageabove 20%. This third alternative was the introduction of acomplementary SF source from the Suspension Atomizing System (SAS), asillustrated in FIGS. 5 and 6.

[0055] Referring to FIGS. 5 and 6, which illustrate the secondary spraysystem, the SAS includes a peristaltic pump 120 (model no. 7553-80,manufactured by Cole-Palmer, Inc., located in Vernon Hills, Ill.), ahead 125 (model no. 7014-20, manufactured by Cole-Palmer, Inc., locatedin Vernon Hills Ill.), and an atomizing probe 140, manufactured byTEKNA, Inc., located in Sherbrooke, Quebec, Canada). The suspension wasprepared by the mixing ratio of 1:1:0.4 by weight of H₂O:SFpowder:dispersing agent, respectively. SF particles having an averageparticle size of 2 microns was obtained from Stackpole, Inc., located inKane, Pa. The dispersing agent was Darvan No. 7, manufactured by R.T.Vanderbilt Company, located in Buena Park, Calif. This particulardispersing agent has been used as a deflocculant for agglomerated SFparticles in liquid suspensions. The SAS was used at a maximum feedingrate of 36.2 g/min., which corresponds to a feed rate of 15.51 g/min SFparticles for a magneto-fluid mixture, which was made by mixing together400 gm of water, 310.2 gm of SF and 14 gm. of DARVAN No. 7.

[0056] Running the SAS system in combination with the thermal spraysystem described in Example 1 (in the manner shown in FIG. 5) with aTeflon coated pan as the substrate, a magnetic sample was obtainedhaving an SF loading of up to 38% by volume. The VSM measurements forthe magnetic sample with the 38% by volume of SF showed the hysteresiscurve illustrated in FIG. 12. The coercivity was 1875 Oe and the sigmavalue was 23.09 emu/g for part (a) in FIG. 1, and the coercivity was1800 Oe and the sigma value was 19.97 emu/g for part (b) in FIG. 1. Thesquareness (Ir/Is) of the SAS sample was 0.526.

[0057] The volume percent of the magnetic particles in the resultingflexible anisotropic magnets can be determined by any reliable methodknown to those skilled in the art. However, the density method and thegravimetric method are preferred. The density method includes measuringthe weight and volume of the flexible magnet and comparing the resultingdensity with the known densities of the polymer, magnetic particles, andthe composite particles. Volume is measured by immersing the flexiblemagnet in a graduated cylinder containing water. Since the secondaryspray system provides 100% of the magnetic particles (e.g., the waterand dispersing agent are vaporized), the volume percent of the magneticparticles in the flexible magnet can then be calculated.

[0058] The gravimetric method includes measuring the weight and volumeof the flexible magnet. The flexible magnet is then heated in thepresence of oxygen at a temperature high enough to oxidize all of thepolymer, i.e., burn the polymer away, but not affect the magneticparticles. The weight and volume of the remaining magnetic particles arethen obtained using the same methods discussed above. The resultingmeasurements can then be compared to the measurements of the flexiblemagnet to calculate the volume percent of the magnetic particles.

What is claimed is:
 1. A method for producing a flexible anisotropicmagnetic coating comprising the steps of: thermal spraying a first spraystream, comprising composite particles of magnetic particles and amatrix material, onto a substrate at a temperature that is above theglass transition or melting point temperature of the matrix material butbelow the Curie temperature of the magnetic particles; and applying amagnetic field to said substrate during the spraying step.
 2. A methodaccording to claim 1, wherein said magnetic particles have an H_(c) ofgreater than about 150 Oe, and wherein said matrix material has amelt-flow index from about 7 to about
 700. 3. A method according toclaim 1, wherein said magnetic particles are selected from the groupconsisting of Sm₂Fe₁₇C, Sm₂Fe₁₂N_(2.7), Sm(CoFeCu)₇, Nd₂Co₁₄B, Nd₂Fe₁₄B,BaFe₁₂O₁₉, CoFe₂O₄, SmCo₅, CoPt, Nd₂Fe₁₄C, Nd₂Fe₁₄N, Fe₃BiNd, SmFe₁₁Ti,SmFe₁₀Mo₂, and mixtures thereof; and said matrix material is selectedfrom the group consisting of ABS, EVA, PEKK, EMAA, PMMA, EAA,polypropylene, polyvinylchloride, polyvinylacetate, nylon, polyethylene,polycarbonate, polystyrene, polyester elastomer, methacryl resin,polyacetal, polyamide resin, thermoplastic polyurethane, JCI,polytherimide, imide based polymers, polyphenylene oxide, fluorplastics,acrylontrile-styrene resin, ionomer resin, vinylchloride vinylacetatecopolymer, polyethylene copolymer and mixtures thereof.
 4. A methodaccording to claim 1, wherein said composite particles compriseparticles of matrix material having magnetic particles incorporatedtherein or thereon.
 5. A method according to claim 4, wherein themagnetic particles have an average particle size from about 1 microns toabout 84 microns, and the particles of the matrix material have anaverage particle size from about 20 microns to about 330 microns.
 6. Amethod according to claim 4, further comprising the step of forming thecomposite particles by incorporating the magnetic particles onto or intothe matrix material particles.
 7. A method according to claim 6, whereinsaid step of forming composite particles includes a mechanofusion step.8. A method according to claim 1, wherein particles of a matrixmaterial, which are free of magnetic particles, are further added tosaid first spray stream.
 9. A method according to claim 1, furthercomprising the step of providing at least one additional spray streamcomprising a magneto-fluid mixture, said at least one additional spraystream intersecting said first spray stream at a predetermined angle tocombine with said first spray stream to coat the substrate.
 10. A methodaccording to claim 9, wherein said at least one additional spray streamis produced by a Suspension Atomizing System.
 11. A method according toclaim 9, wherein said magneto-fluid mixture comprises magneticparticles, a vaporizable fluid, and a dispersing agent.
 12. A methodaccording to claim 1, wherein said substrate is a removable mold.
 13. Anarticle of manufacture obtained from the method according to claim 1.14. An article of manufacture obtained from the method according toclaim
 9. 15. An article of manufacture obtained from the methodaccording to claim
 12. 16. An article of manufacture havingmagnetocrystalline anisotropic magnetic energy, comprising: a substrate;and a flexible magnetic coating fixedly attached to said substrate, saidcoating comprising magnetic particles incorporated into or onto matrixmaterial and thermally sprayed onto said substrate in the presence of anapplied magnetic field at said substrate.
 17. An article of manufactureaccording to claim 16, wherein said magnetic particles have an H_(c) ofgreater than about 150 Oe, and wherein said matrix material has amelt-flow index from about 7 to about
 700. 18. An article according toclaim 16, wherein said magnetic particles are selected from the groupconsisting of Sm₂Fe₁₇C, Sm₂Fe₁₂N_(2.7), Sm(CoFeCu)₇, Nd₂Co₁₄B, Nd₂Fe₁₄B,BaFe₁₂O₁₉, CoFe₂O₄, SmCo₅, CoPt, Nd₂Fe₁₄C, Nd₂Fe₁₄N, Fe₃BiNd, SmFe₁₁Ti,SmFe₁₀V₂, SmFe₁₀Mo₂, and mixtures thereof, and said matrix material isselected from the group consisting of ABS, EVA, PEKK, EMAA, PMMA, EAA,polypropylene, polyvinylchloride, polyvinylacetate, nylon, polyethylene,polycarbonate, polystyrene, polyester elastomer, methacryl resin,polyacetal, polyamide resin, thermoplastic polyurethane, JCI,polytherimide, imide based polymers, polyphenylene oxide, fluorplastics,acrylontrile-styrene resin, ionomer resin, vinylchloride vinylacetatecopolymer, polyethylene copolymer and mixtures thereof.
 19. An articleaccording to claim 16, wherein said flexible magnetic coating comprisesfrom about 8% to about 38%, by volume of the coating, of magneticparticles.
 20. An article according to claim 16, wherein at least onesection of said article has an easy magnetic axis.
 21. An articleaccording to claim 16, wherein said article has a coercivity of greaterthan about 2200 Oe.
 22. A flexible anisotropic magnet, comprisingmagnetic particles dispersed within a matrix material and formed bythermal spraying onto a removable mold in the presence of an appliedmagnetic field at the mold.
 23. A flexible anisotropic magnet accordingto claim 22, wherein said magnetic particles have an H_(c) of greaterthan about 150Oe, and wherein said matrix material has a meltflow indexfrom about 7 to about
 700. 24. A flexible anisotropic magnet accordingto claim 22, wherein said magnetic particles are selected from the groupconsisting of Sm₂Fe₁₇C, Sm₂Fe₁₂N₂₇, Sm(CoFeCu)₇, Nd₂Co₁₄B, Nd₂Fe₁₄B,BaFe₁₂O₁₉, CoFe₂O₄, SmCo₅, CoPt, Nd₂Fe₁₄C, Nd₂Fe₁₄N, Fe₃BiNd, SmFe₁₁Ti,SmFe₁₀V₂, SmFe₁₀Mo₂, and mixtures thereof; and said matrix material isselected from the group consisting of ABS, EVA, PEKK, EMAA, PMMA, EAA,polypropylene, polyvinylchloride, polyvinylacetate, nylon, polyethylene,polycarbonate, polystyrene, polyester elastomer, methacryl resin,polyacetal, polyamide resin, thermoplastic polyurethane, JCI,polytherimide, imide based polymers, polyphenylene oxide, fluorplastics,acrylontrile-styrene resin, ionomer resin, vinylchloride vinylacetatecopolymer, polyethylene copolymer and mixtures thereof.
 25. A flexibleanisotropic magnet according to claim 22, comprising from about 8% toabout 38%, by volume, of magnetic particles.
 26. A flexible anisotropicmagnet according to claim 22, wherein at least one section of saidmagnet has an easy magnetic axis.
 27. A flexible anisotropic magnetaccording to claim 22, wherein said magnet has a coercivity of greaterthan about 2200 Oe.